1. Field of the Invention
The present invention relates to a novel polyurethane resin. More particularly, the present invention relates to a polyurethane resin which contains carbodiimide groups in the molecular chain and which shows water resistance stably over a long period of time.
2. Prior Art
Polyurethane resins have high elasticity and are in use in various applications such as molded product, thread, coating film, etc. Meanwhile, they are inferior in chemical resistance, heat resistance and water resistance and, therefore, have not been used in the applications where said properties are required, such as hose and packing for automobile engine and peripherals.
Of various polyurethane resins, polyester polyurethane resins obtained from a polyester polyol contain a large number of ester bonds in the molecule and, when used under high-temperature and high-humidity conditions, give rise to hydrolysis of ester bonds and consequent molecular weight reduction, inviting deterioration of strength and elastic modulus.
In order to alleviate the above-mentioned problems of polyurethane resin caused by ester bond hydrolysis and consequent molecular weight reduction, it has heretofore been proposed to add an additive to a polyurethane resin. An example of such proposals is to add epoxy, oxazoline, carbodiimide or the like to a polyurethane resin to allow the additive to trap the carboxyl group formed by hydrolysis of the polyurethane resin and extend the life of the polyurethane resin.
However, since the additive is simply added to a polyurethane resin, the additive bleeds out onto the surface of the polyurethane resin; when the additive is added to a polyurethane resin which comes into contact with a solvent or an oil, the additive is extracted by the solvent or oil, inviting the performance deterioration of the solvent or oil and the reduction in hydrolysis resistance of the polyurethane resin. Therefore, the effect of addition of the above additive to polyurethane resin as a hydrolysis resistant is not fully satisfactory.
Moreover, since the above additive is generally a powdery solid of high melting point, dispersion thereof in polyurethane resin needs a very complicated apparatus and a fairly long time.